Self-activated superhydrophilic green ZnIn2S4 realizing solar-driven overall water splitting: close-to-unity stability for a full daytime

Engineering an efficient semiconductor to sustainably produce green hydrogen via solar-driven water splitting is one of the cutting-edge strategies for carbon-neutral energy ecosystem. Herein, a superhydrophilic green hollow ZnIn2S4 (gZIS) was fabricated to realize unassisted photocatalytic overall water splitting. The hollow hierarchical framework benefits exposure of intrinsically active facets and activates inert basal planes. The superhydrophilic nature of gZIS promotes intense surface water molecule interactions. The presence of vacancies within gZIS facilitates photon energy utilization and charge transfer. Systematic theoretical computations signify the defect-induced charge redistribution of gZIS enhancing water activation and reducing surface kinetic barriers. Ultimately, the gZIS could drive photocatalytic pure water splitting by retaining close-to-unity stability for a full daytime reaction with performance comparable to other complex sulfide-based materials. This work reports a self-activated, single-component cocatalyst-free gZIS with great exploration value, potentially providing a state-of-the-art design and innovative aperture for efficient solar-driven hydrogen production to achieve carbon-neutrality.

where α is the absorption coefficient, α0 is the Urbach constant, hν is the incident photon energy and Eu is the Urbach energy (which can be calculated as the reciprocal of the gradient in the linearized Urbach equation).Consistent calculation is performed by extrapolating the transition energy from Tauc plot to determine the location of defect state as displayed in Supplementary Fig. 10a.It can be shown that both calculations, Urbach energy and transition energy, converge to the same position of the defect level, implying an accurate finding is achieved.As presented in Supplementary Fig. 11, pristine ZIS exhibits a singular PL peak at 536 nm (approximately 2.31 eV), corresponding to its Eg determined through the KM relationship.Conversely, gZIS demonstrates two distinct PL peaks, in which the first peak at 483 nm (around 2.57 eV) arises from the intrinsic band-to-band radiative transition of photoexcited electrons from the CB to the VB, closely aligning with the Eg derived.The second peak at 530 nm (approximately 2.34 eV) originates from the extrinsic sub-band defect state introduced by Sv to the ground state.This value closely corresponds to the defect energy calculated using the Urbach's equation and transition energy.Supplementary Fig. 12. UPS spectra of (a) ZIS and (b) gZIS for determining the valence band energy.
The valence band energy (EVB) with respect to vacuum was evaluated according to the formula: 4, 5, 6, 7 Thus, the respective Gibbs free energy of each step could be evaluated by referring to standard Gibbs free energy of water splitting (∆G 2H 2 O→O 2 +2H 2 ) of 4.92 eV, 2 which is summarized as:

Fig. 11 .
(a) Band structures and (b) PL spectra for ZIS and gZIS sample.

Supplementary Fig. 13 . 2 E H 2 )Supplementary Fig. 16 .Supplementary Fig. 17 .
S5) whereby hν represents the incident photon energy of He light source of 21.22 eV, Ecut denotes the electron cut-off edge, and Efe is the Fermi edge of the samples.Following that, unit conversions were applied based on the relationship between vacuum energy (Evac) and NHE potential (ENHE) as in 0 V vs. NHE is equal to -4.44 eV in vacuum, as well as a pH correction factor of 0.059 pH to convert to NHE scale at pH 7. In short, ZIS possesses an EVB of 6.19 eV below vacuum (1.34 V vs. NHE at pH 7) and gZIS exhibits an EVB position of 6.55 eV below vacuum (1.70 V vs. NHE at pH 7).Theoretical work function (WF) of (a) ZIST and (b) gZIST.Supplementary Fig. 14.Potential line profiles of (a) ZIS and (b) gZIS, with insets showing the respective x-y scan area.The white line indicates the longitudinal scan direction.(c) Illustration of the estimated WF positions of ZIS and gZIS with respect to FTO.The relative work function (WF) of the samples can be estimated by measuring the contact potential difference (CPD) between the sample and conductive reference (i.e., FTO) across the interfacial boundary.As presented in Supplementary Fig.14, the CPD values were measured by sweeping through the sample with a biased AFM probe, by which the counter bias voltage used in neutralizing the electric field was recorded.In this regard, gZIS possesses a comparatively lower CPD value (∆V = 322.7 mV) than that of ZIS (∆V = 424.0mV).By taking FTO as a conductive reference, the local variation of WF in the samples could be attained and the changes in relative WF of the samples could be feasibly compared.8Thus, it could be observed that gZIS experiences reduction in WF, accompanied by the uplift of Fermi level to facilitate photogenerated electron transition.Supplementary Fig. 15.Free energy diagram for HER for (a) S atom at [001] facet of ZIST, (b) S atom at [110] facet of ZIST, (c) S atom at [001] facet of gZIST, (d) Sv position at [001] facet of gZIST, and (e) S atom at [110] facet of gZIST.For single HER reaction, the corrected H * adsorption Gibbs free energy (∆G H * ) at U = 0 could be calculated via : 9 ∆G H * = ∆E H * + (∆ZPE − T∆S) H * = (E H * − E * − 1 + 0.24 (S6) by which ∆E H * is the differential adsorption energy of H * on surface slab, E * is the clean surface energy and E H 2 is the energy of free H2.Optimized structural model of adsorbed HO*, O* and HOO* onto (a) Zn-ZIST, (b) Zn-gZIST and (c) Sv-gZIST.Free energies of (a) Zn-ZIST, (b) Zn-gZIST and (c) Sv-gZIST with U = 0 [no applied bias] and U = 1.23 V [standard equilibrium potential of OER].Rate determining step (RDS) is marked as red in each of the sub-figure.The OER process could be evaluated following the four elementary steps below which HO*, O* and HOO* adsorbed intermediate onto active surface slab (*).* +H 2 O ⇌ HO * + H + + e − (S7) HO * ⇌ O * + H + + e − (S8)

Table 5 .
, E O * and E HOO * dictate the energy of surfaces with adsorbed HO * , O * and HOO * , Comparison of AQY at 420 nm monochromatic light and STH performance (pure water) for gZIS with other single-component sulfide-based photocatalysts. *